The present invention relates generally to digital cameras, and more particularly, to a digital camera capturing technique for documents.
Projecting one or more points onto an object and detecting the projected points using an autofocus detector is the basis of so-called active (but nonacoustic) autofocus mechanisms in typical point-and-shoot cameras. Cameras can reliably determine the focal distance to a small area on an object, but cannot determine if an object is imaged obliquely, as can be the case with handheld exposures of documents. Cameras can indicate to the user that an object is too close to focus upon, but do not indicate that the depth of focus is inadequate to image an entire object. Determining the adequacy of the depth of focus of an entire object represents a complex problem that is typically left to the user. However, manual procedures, such as stopped-down viewfinder techniques used by skilled photographers, are very inconvenient and prone to error particularly in low-contrast situations in a stopped-down viewfinder.
In one embodiment, the present invention is incorporated into a digital camera to permit compensation to be performed for the following effects in the handheld capture of documents at close range:
the optical axis of the camera is not aligned with a normal to the surface of the document;
the depth-of-field for optics used for capture within 0.5-1 meter from the focal plane of the image sensor is limited, so proper focus is critical; and
typical documents may not be planar, for example, pages bound in a book.
This embodiment extends the concept of auto-focus for cameras to auto-focus and auto-orientation, and provides an indication of the orientation of the optical axis with respect to the normal to the plane of the document and an indication of the degree of planarity of the document (e.g., including page curl of the document).
A typical active autofocus camera employs an infrared emitter and a photodetector arranged on either side of the optical axis. As the shutter release is pressed part-way, a highly-collimated IR beam is projected (usually as a vertical bar) on the subject, and the position of the beam is imaged on the detector. The measurement of range relies on parallax between the emitter and detector.
In one embodiment, the present invention involves projecting beams of visible or, preferably, infrared (IR) light on the document to be captured in a predetermined pattern (e.g., a spot pattern) as the shutter release is pressed part-way. The projected pattern is designed to provide information of range, orientation, and planarity of the document. The pattern is received by the image sensor in the digital camera and stored (or processed) immediately before the capture of the subject image using the image sensor. Using IR illumination provides an invisible measurement and uses the sensitivity of, for example, commercially available CCD and CMOS sensors to IR wavelengths. High intensity IR-LEDs are commercially available for use as emitters.
A pattern to provide range and alignment can be provided by projecting three beams of light in a pattern. More robust patterns for measuring planarity of the subject can employ more than three beams of light, for example, to provide a grid of multiple cells.
In one embodiment, a method for a digital camera capturing technique includes determining an orientation of an object (e.g., a document) relative to an optical axis of the digital camera, and determining a distance of the object from the digital camera. The method also includes determining a planarity of the object. The method further includes projecting a pattern on the object, and detecting the projected pattern, which is then processed to determine the orientation of the object relative to the optical axis of the digital camera, the distance of the object from the digital camera, and the planarity of the object.